Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support to the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is to be finalized as Release 8 (LTE Rel. 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP), and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmit power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques, and a highly efficient control signaling structure is achieved in LTE Rel. 8/9.
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink QoS, cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMES/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
QoS Control
Efficient Quality of Service (QoS) support is seen as a basic requirement by operators for LTE. In order to allow best in class user experience, while on the other hand optimizing the network resource utilization, enhanced QoS support should be integral part of the new system.
Aspects of QoS support is currently being under discussion within 3GPP working groups. Essentially, the QoS design for System Architecture Evolution (SAE)/LTE is based on the QoS design of the current UMTS system reflected in 3GPP TR 25.814, “Physical layer aspects for evolved Universal Terrestrial Radio Access (UTRA)”, v.7.1.0 (available at http://www.3gpp.org and incorporated herein by reference). The agreed SAE Bearer Service architecture is depicted in FIG. 5. The definition of a bearer service as given in 3GPP TR 25.814 may still be applicable:
“A bearer service includes all aspects to enable the provision of a contracted QoS. These aspects are among others the control signaling, user plane transport and QoS management functionality”.
In the new SAE/LTE architecture the following new bearers have been defined: the SAE Bearer service between the mobile terminal (User Equipment—UE) and the serving gateway, the SAE Radio Bearer on the radio access network interface between mobile terminal and eNodeB as well as the SAE Access Bearer between the eNodeB and the serving gateway.
The SAE Bearer Service provides:                QoS-wise aggregation of IP end-to-end-service flows;        IP header compression (and provision of related information to UE);        User Plane (UP) encryption (and provision of related information to UE);        if prioritized treatment of end-to-end-service signaling packets is required an additional SAE Bearer Service can be added to the default IP service;        provision of mapping/multiplexing information to the UE;        provision of accepted QoS information to the UE.        
The SAE Radio Bearer Service provides:                transport of the SAE Bearer Service data units between eNodeB and UE according to the required QoS;        linking of the SAE Radio Bearer Service to the respective SAE Bearer Service.        
The SAE Access Bearer Service provides:                transport of the SAE Bearer Service data units between serving gateway and eNodeB according to the required QoS;        provision of aggregate QoS description of the SAE Bearer Service towards the eNodeB;        linking of the SAE Access Bearer Service to the respective SAE Bearer Service.        
In 3GPP TR 25.814 a one-to-one mapping between an SAE Bearer and an SAE Radio Bearer. Furthermore there is a one-to-one mapping between a radio bearer (RB) and a logical channel. From that definition it follows that a SAE Bearer, i.e. the corresponding SAE Radio Bearer and SAE Access Bearer, is the level of granularity for QoS control in an SAE/LTE access system. Packet flows mapped to the same SAE Bearer receive the same treatment.
For LTE there will be two different SAE bearer types: the default SAE bearer with a default QoS profile, which is configured during initial access and the dedicated SAE bearer (SAE bearers may also be referred to as SAE bearer services) which is established for services requiring a QoS profile which is different from the default one.
The default SAE bearer is an “always on” SAE bearer that can be used immediately after LTE_IDLE to LTE_ACTIVE state transition. It carries all flows which have not been signaled a Traffic Flow Template (TFT). The Traffic Flow Template is used by serving gateway to discriminate between different user payloads. The Traffic Flow Template incorporates packet filters such as QoS. Using the packet filters the serving gateway maps the incoming data into the correct PDP Context (Packet Data Protocol Context). For the default SAE bearer, several service data flows can be multiplexed. Unlike the default SAE Bearer, the dedicated SAE Bearers are aimed at supporting identified services in a dedicated manner, typically to provide a guaranteed bit-rate. Dedicated SAE bearers are established by the serving gateway based on the QoS information received in Policy and Charging Control (PCC) rules from evolved packet core when a new service is requested. A dedicated SAE bearer is associated with packet filters where the filters match only certain packets. A default SAE bearer is associated with “match all” packet filters for uplink and downlink. For uplink handling the serving gateway builds the Traffic Flow Template filters for the dedicated SAE bearers. The UE maps service data flows to the correct bearer based on the Traffic Flow Template, which has been signaled during bearer establishment. As for the default SAE Bearer, also for the dedicated SAE Bearer several service data flows can be multiplexed.
The QoS Profile of the SAE bearer is signaled from the serving gateway to the eNodeB during the SAE bearer setup procedure. This profile is then used by the eNodeB to derive a set of Layer 2 QoS parameters, which will determine the QoS handling on the air interface. The Layer 2 QoS parameters are input to the scheduling functionality. The parameters included in the QoS profile signaled on S1 interface from serving gateway to eNodeB are currently under discussion. Most likely the following QoS profile parameters are signaled for each SAE bearer: Traffic Handling Priority, Maximum Bit-rate, Guaranteed Bit-rate. In addition, the serving gateway signals to the eNodeB the Allocation and Retention Priority for each user during initial access.
Uplink Access Scheme for LTE
For uplink transmission, power-efficient user-terminal transmission is necessary to maximize coverage. Single-carrier transmission combined with FDMA (Frequency Division Multiple Access) with dynamic bandwidth allocation has been chosen as the evolved UTRA uplink transmission scheme. The main reason for the preference for single-carrier transmission is the lower peak-to-average power ratio (PAPR), compared to multi-carrier signals (OFDMA—Orthogonal Frequency Division Multiple Access), and the corresponding improved power-amplifier efficiency and assumed improved coverage (higher data rates for a given terminal peak power). During each time interval, eNodeB assigns users a unique time/frequency resource for transmitting user data thereby ensuring intra-cell orthogonality. An orthogonal access in the uplink promises increased spectral efficiency by eliminating intra-cell interference. Interference due to multipath propagation is handled at the base station (eNodeB), aided by insertion of a cyclic prefix in the transmitted signal.
The basic physical resource used for data transmission consists of a frequency resource of size BWgrant during one time interval, e.g. a sub-frame of 0.5 ms, onto which coded information bits are mapped. It should be noted that a sub-frame, also referred to as transmission time interval (TTI), is the smallest time interval for user data transmission. It is however possible to assign a frequency resource BWgrant over a longer time period than one TTI to a user by concatenation of sub-frames.
The frequency resource can either be in a localized or distributed spectrum as illustrated in FIG. 3 and FIG. 4. As can be seen from FIG. 3, localized single-carrier is characterized by the transmitted signal having a continuous spectrum that occupies a part of the total available spectrum. Different symbol rates (corresponding to different data rates) of the transmitted signal imply different bandwidths of a localized single-carrier signal.
On the other hand, as shown in FIG. 4, distributed single-carrier is characterized by the transmitted signal having a non-continuous (“comb-shaped”) spectrum that is distributed over system bandwidth. Note that, although the distributed single-carrier signal is distributed over the system bandwidth, the total amount of occupied spectrum is, in essence, the same as that of localized single-carrier. Furthermore, for higher/lower symbol rate, the number of “comb-fingers” is increased/reduced, while the “bandwidth” of each “comb finger” remains the same.
At first glance, the spectrum in FIG. 4 may give the impression of a multi-carrier signal where each comb-finger corresponds to a “sub-carrier”. However, from the time-domain signal-generation of a distributed single-carrier signal, it should be clear that what is being generated is a true single-carrier signal with a corresponding low peak-to-average power ratio. The key difference between a distributed single-carrier signal versus a multi-carrier signal, such as e.g. OFDM (Orthogonal Frequency Division Multiplex), is that, in the former case, each “sub-carrier” or “comb finger” does not carry a single modulation symbol. Instead each “comb-finger” carries information about all modulation symbols. This creates a dependency between the different comb-fingers that leads to the low-PAPR characteristics. It is the same dependency between the “comb fingers” that leads to a need for equalization unless the channel is frequency-non-selective over the entire transmission bandwidth. In contrast, for OFDM equalization is not needed as long as the channel is frequency-non-selective over the sub-carrier bandwidth.
Distributed transmission can provide a larger frequency diversity gain than localized transmission, while localized transmission more easily allows for channel-dependent scheduling. Note that, in many cases the scheduling decision may decide to give the whole bandwidth to a single user equipment to achieve high data rates.
Uplink Scheduling Scheme for LTE
The uplink scheme allows for both scheduled access, i.e. controlled by eNodeB, and contention-based access.
In case of scheduled access the user equipment is allocated a certain frequency resource for a certain time (i.e. a time/frequency resource) for uplink data transmission. However, some time/frequency resources can be allocated for contention-based access. Within these time/frequency resources, user equipments can transmit without first being scheduled. One scenario where user equipment is making a contention-based access is for example the random access, i.e. when user equipment is performing initial access to a cell or for requesting uplink resources.
For the scheduled access eNodeB scheduler assigns a user a unique frequency/time resource for uplink data transmission. More specifically the scheduler determines                which user equipment(s) that is (are) allowed to transmit,        which physical channel resources (frequency),        Transport format (Transport Block Size (TBS) and Modulation Coding Scheme (MCS)) to be used by the mobile terminal for transmission        
The allocation information is signaled to the user equipment via a scheduling grant, sent on the so-called L1/L2 control channel. For simplicity, this downlink channel is referred to the “uplink grant channel” in the following.
A scheduling grant message (also referred to as an resource assignment herein) contains at least information which part of the frequency band the user equipment is allowed to use, the validity period of the grant, and the transport format the user equipment has to use for the upcoming uplink transmission. The shortest validity period is one sub-frame. Additional information may also be included in the grant message, depending on the selected scheme. Only “per user equipment” grants are used to grant the right to transmit on the Uplink Shared Channel UL-SCH (i.e. there are no “per user equipment per RB” grants). Therefore the user equipment needs to distribute the allocated resources among the radio bearers according to some rules, which will be explained in detail in the next section.
Unlike in HSUPA there is no user equipment based transport format selection. The base station (eNodeB) decides the transport format based on some information, e.g. reported scheduling information and QoS information, and user equipment has to follow the selected transport format. In HSUPA eNodeB assigns the maximum uplink resource and user equipment selects accordingly the actual transport format for the data transmissions.
Uplink data transmissions are only allowed to use the time-frequency resources assigned to the user equipment through the scheduling grant. If the user equipment does not have a valid grant, it is not allowed to transmit any uplink data. Unlike in HSUPA, where each user equipment is always allocated a dedicated channel there is only one uplink data channel shared by multiple users (UL-SCH) for data transmissions.
To request resources, the user equipment transmits a resource request message to the eNodeB. This resources request message could for example contain information on the buffer status, the power status of the user equipment and some Quality of Services (QoS) related information. This information, which will be referred to as scheduling information, allows eNodeB to make an appropriate resource allocation. Throughout the document it's assumed that the buffer status is reported for a group of radio bearers. Of course other configurations for the buffer status reporting are also possible. Since the scheduling of radio resources is the most important function in a shared channel access network for determining Quality of Service, there are a number of requirements that should be fulfilled by the uplink scheduling scheme for LTE in order to allow for an efficient QoS management (see 3GPP RAN WG#2 Tdoc. R2-R2-062606, “QoS operator requirements/use cases for services sharing the same bearer”, by T-Mobile, NTT DoCoMo, Vodafone, Orange, KPN; available at http://www.3gpp.org/and incorporated herein by reference):                Starvation of low priority services should be avoided        Clear QoS differentiation for radio bearers/services should be supported by the scheduling scheme        The uplink reporting should allow fine granular buffer reports (e.g. per radio bearer or per radio bearer group) in order to allow the eNodeB scheduler to identify for which Radio Bearer/service data is to be sent.        It should be possible to make clear QoS differentiation between services of different users        It should be possible to provide a minimum bit-rate per radio bearer        
As can be seen from above list one essential aspect of the LTE scheduling scheme is to provide mechanisms with which the operator can control the partitioning of its aggregate cell capacity between the radio bearers of the different QoS classes. The QoS class of a radio bearer is identified by the QoS profile of the corresponding SAE bearer signaled from serving gateway to eNodeB as described before. An operator can then allocate a certain amount of its aggregate cell capacity to the aggregate traffic associated with radio bearers of a certain QoS class.
The main goal of employing this class-based approach is to be able to differentiate the treatment of packets depending on the QoS class they belong to. For example, as the load in a cell increases, it should be possible for an operator to handle this by throttling traffic belonging to a low-priority QoS class. At this stage, the high-priority traffic can still experience a low-loaded situation, since the aggregate resources allocated to this traffic is sufficient to serve it. This should be possible in both uplink and downlink direction.
One benefit of employing this approach is to give the operator full control of the policies that govern the partitioning of the bandwidth. For example, one operator's policy could be to, even at extremely high loads, avoid starvation of traffic belonging to its lowest priority QoS Class. The avoidance of starvation of low priority traffic is one of the main requirements for the uplink scheduling scheme in LTE. In current UMTS Release 6 (HSUPA) scheduling mechanism the absolute prioritization scheme may lead to starvation of low priority applications. E-TFC selection (Enhanced Transport Format Combination selection) is done only in accordance to absolute logical channel priorities, i.e. the transmission of high priority data is maximized, which means that low priority data is possibly starved by high priority data. In order to avoid starvation the eNodeB scheduler must have means to control from which radio bearers a user equipment transmits data. This mainly influences the design and use of the scheduling grants transmitted on the L1/L2 control channel in downlink. In the following the details of the uplink rate control procedure in LTE is outlined.
Medium Access Control (MAC) and MAC Control Elements
The MAC layer is the lowest sub-layer in the Layer 2 architecture of the LTE radio protocol stack (see 3GPP TS 36.321, “Medium Access Control (MAC) protocol specification”, version 8.7.0, in particular sections 4.2, 4.3, 5.4.3 and 6, available at http//www.3gpp.org and incorporated in its entirety herein by reference). The connection to the physical layer below is through transport channels, and the connection to the RLC layer above is through logical channels. The MAC layer performs multiplexing and demultiplexing between logical channels and transport channels. The MAC layer in the transmitting side (in the following examples the user equipment) constructs MAC PDUs, also referred to as transport blocks, from MAC SDUs received through logical channels, and the MAC layer in the receiving side recovers MAC SDUs from MAC PDUs received through transport channels.
In the multiplexing and demultiplexing entity, data from several logical channels can be (de)multiplexed into/from one transport channel. The multiplexing entity generates MAC PDUs from MAC SDUs when radio resources are available for a new transmission. This process includes prioritizing the data from the logical channels to decide how much data and from which logical channel(s) should be included in each MAC PDU. Please note that the process of generating MAC PDUs in the user equipment is also referred to a logical channel prioritization (LCP) in the 3GPP terminology.
The demultiplexing entity reassembles the MAC SDUs from MAC PDUs and distributes them to the appropriate RLC entities. In addition, for peer-to-peer communication between the MAC layers, control messages called ‘MAC Control Elements’ can be included in the MAC PDU.
A MAC PDU primarily consists of the MAC header and the MAC payload (see 3GPP TS 36.321, section 6). The MAC header is further composed of MAC sub-headers, while the MAC payload is composed of MAC Control Elements, MAC SDUs and padding. Each MAC sub-header consists of a Logical Channel ID (LCID) and a Length (L) field. The LCID indicates whether the corresponding part of the MAC payload is a MAC Control Element, and if not, to which logical channel the related MAC SDU belongs. The L field indicates the size of the related MAC SDU or MAC Control Element. As already mentioned above, MAC Control Elements are used for MAC-level peer-to-peer signaling, including delivery of BSR information and reports of the UE's available power in the uplink, and in the downlink DRX commands and timing advance commands. For each type of MAC Control Element, one special LCID is allocated. An example for a MAC PDU is shown in FIG. 6.
Power Control
Uplink transmitter power control in a mobile communication system serves the purpose of balancing the need for sufficient transmitter energy per bit to achieve the required QoS against the need to minimize interference to other users of the system and to maximize the battery life of the user equipment. In achieving this, the uplink power control has to adapt to the characteristics of the radio propagation channel, including path loss, shadowing and fast fading, as well as overcoming interference from other users within the same cell and neighboring cells. The role of the Power Control (PC) becomes decisive to provide the required SINR (Signal-to-Interference plus Noise Ratio) while controlling at the same time the interference caused to neighboring cells. The idea of classic PC schemes in uplink is that all users are received with the same SINR, which is known as full compensation. As an alternative, the 3GPP has adopted the use of Fractional Power Control (FPC) for LTE Rel. 8/9. This new functionality makes users with a higher path-loss operate at a lower SINR requirement so that they will more likely generate less interference to neighboring cells.
The power control scheme provided in LTE Rel. 8/9 employs a combination of open-loop and closed-loop control. A mode of operation involves setting a coarse operating point for the transmission power density spectrum by open-loop means based on path-loss estimation. Faster operation can then be applied around the open-loop operating point by closed-loop power control. This controls interference and fine-tunes the power settings to suit the channel conditions including fast fading.
With this combination of mechanisms, the power control scheme in LTE Rel. 8/9 provides support for more than one mode of operation. It can be seen as a toolkit for different power control strategies depending on the deployment scenario, the system load and operator preference.
The detailed power control formulae are specified in LTE Rel. 8/9 for the Physical Uplink Shared Channel (PUSCH), Physical Uplink Control Channel (PUCCH) and the Sounding Reference Signals (SRS) in section 5.1 in 3GPP TS 36.213, “Physical layer procedures”, version 8.8.0, available at http://www.3gpp.org and incorporated herein by reference. The formula for each of these uplink signals follows the same basic principles; in all cases they can be considered as a summation of two main terms: a basic open-loop operating point derived from static or semi-static parameters signaled by the eNodeB, and a dynamic offset updated from sub-frame to sub-frame.
The basic open-loop operating point for the transmit power per resource block depends on a number of factors including the inter-cell interference and cell load. It can be further broken down into two components, a semi-static base level P0, further comprised of a common power level for all user equipments in the cell (measured in dBm) and a UE-specific offset, and an open-loop path-loss compensation component. The dynamic offset part of the power per resource block can also be further broken down into two components, a component dependent on the MCS and explicit Transmitter Power Control (TPC) commands.
The MCS-dependent component (referred to in the LTE specifications as ΔTF, where TF stands for “Transport Format”) allows the transmitted power per resource block to be adapted according to the transmitted information data rate.
The other component of the dynamic offset is the UE-specific TPC commands. These can operate in two different modes: accumulative TPC commands (available for PUSCH, PUCCH and SRS) and absolute TPC commands (available for PUSCH only). For the PUSCH, the switch between these two modes is configured semi-statically for each UE by RRC signaling—i.e. the mode cannot be changed dynamically. With the accumulative TPC commands, each TPC command signals a power step relative to the previous level. Uplink transmitter power control in a mobile communication system serves the purpose of balancing the need for sufficient transmitter energy per bit to achieve the required QoS against the need to minimize interference to other users of the system and to maximize the battery life of the user equipment.
In achieving this, the uplink power control has to adapt to the characteristics of the radio propagation channel, including path loss, shadowing and fast fading, as well as overcoming interference from other users within the same cell and neighboring cells.
The setting of the UE Transmit power PPUSCH [dBm] for the PUSCH transmission in reference sub-frame i is defined by (see section 5.1.1.1 of 3GPP TS 36.213):PPUSCH(i)=min{PCMAX,10 log10(MPUSCH(i))+PO_PUSCH(j)+α(j)·PL+ΔTF(i)+f(i)}   Equation 1                PCMAX is the maximum UE transmit power chosen by UE in the given range (see below); MPUSCH is the number of allocated physical resource blocks (PRBs). The more PRBs are allocated, the more uplink transmit power is allocated.        PO_PUSCH (j) indicates the base transmission power signaled by RRC. For semi-persistent scheduling (SPS) and dynamic scheduling this is the sum of a cell specific nominal component PO_NOMINAL_PUSCH (j)ε[−126, . . . , 24] and a UE specific component PO_UE_PUSCH (j)ε[127, . . . , −96]. For RACH message 3: Offset from preamble transmission power        α denotes a cell-specific parameter (that is broadcast on system information). This parameter indicates how much path-loss PL is compensated. α=1 means the received signal level at eNodeB is same regardless of the user equipment's position. For SPS and dynamic scheduling αε{0, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1}, and for the case of RACH Message 3, α(j)=1.        PL is the UE path-loss derived at the user equipments based on Reference Signal Received Power (RSRP) measurement and signaled Reference Signal (RS) transmission power. PL can be defined as PL=reference signal power−higher layer filtered RSRP.        ΔTF is a modulation and coding scheme (transport format) dependent power offset.        ƒ(i) is a function of the closed loop power control commands signaled from the eNodeB to the UE. ƒ( ) represents accumulation in case of accumulative TPC commands. Whether closed loop commands are relative accumulative or absolute is configured by higher layers. For the accumulative TPC commands two sets of power step values are provided: (−1,1) dB for DCI format 3A and (−1, 0+1, +3)dB for DCI format 3. The set of values which can be signaled by absolute TPC commands is (−4, −1, 1, 4) dB indicated by DCI format 3.Power Headroom Reporting        
In order to assist the eNodeB to schedule the uplink transmission resources to different user equipments in an appropriate way, it is important that the user equipment can report its available power headroom to eNodeB.
The eNodeB can use the power headroom reports to determine how much more uplink bandwidth per sub-frame a user equipment is capable of using. This helps to avoid allocating uplink transmission resources to user equipments which are unable to use them in order to avoid a waste of resources.
The range of the power headroom report is from +40 to −23 dB (see 3GPP TS 36.133, “Requirements for support of radio resource management”, version 8.7.0, section 9.1.8.4, available at http//www.3gpp.org and incorporated in its entirety herein by reference). The negative part of the range enables the user equipment to signal to the eNodeB the extent to which it has received an UL grant which would require more transmission power than the UE has available. This would enable the eNodeB to reduce the size of a subsequent grant, thus freeing up transmission resources to allocate to other UEs.
A power headroom report can only be sent in sub-frames in which a UE has an UL grant. The report relates to the sub-frame in which it is sent. A number of criteria are defined to trigger a power headroom report. These include:                A significant change in estimated path loss since the last power headroom report        More than a configured time has elapsed since the previous power headroom report        More than a configured number of closed-loop TPC commands have been implemented by the UE        
The eNodeB can configure parameters to control each of these triggers depending on the system loading and the requirements of its scheduling algorithm. To be more specific, RRC controls power headroom reporting by configuring the two timers periodicPHR-Timer and prohibitPHR-Timer, and by signalling dl-PathlossChange which sets the change in measured downlink pathloss to trigger a power headroom report.
The power headroom report is send as a MAC Control Element. It consists of a single octet where the two highest bits are reserved and the six lowest bits represent the dB values mentioned above in 1 dB steps. The structure of the MAC Control Element is shown in FIG. 7.
The UE power headroom PH [dB] valid for sub-frame i is defined by (see section 5.1.1.2 of 3GPP TS 36.213):PH(i)=PCMAX−{10·log10(MPUSCH(i))+PO_PUSCH(j)+α(j)·PL+ΔTF(i)+f(i)}   Equation 2
The power headroom is rounded to the closest value in the range [40; −23] dB with steps of 1 dB. PCMAX is the total maximum UE transmit power (or total maximum transmit power of the user equipment) and is a value chosen by user equipment in the given range of PCMAX_L and PCMAX_H based on the following constraints:PCMAX_L≦PCMAX≦PCMAX_H PCMAX_L=min(PEMAX−ΔTC,PPowerClass−MPR−AMPR−ΔTC)PCMAX_H=min(PEMAX,PPowerclass)
PEMAX is the value signaled by the network and ΔTc, MPR and AMPR (also denoted A-MPR—Additional Maximum Power Reduction) are specified in 3GPP TS 36.101, “Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio transmission and reception”, version 8.7.0, section 6.2 available at http//www.3gpp.org and incorporated herein by reference.
MPR is a power reduction value, the so-called Maximum Power Reduction, used to control the Adjacent Channel Leakage Power Ratio (ACLR) associated with the various modulation schemes and the transmission bandwidth. An adjacent channel may be for example either another Evolved Universal Terrestrial Radio Access (E-UTRA) channel or an UTRA channel. The maximum allowed power reduction (MPR) is also defined in 3GPP TS 36.101. It is different depending on channel bandwidth and modulation scheme. The user equipment's reduction may be less than this maximum allowed power reduction (MPR) value. 3GPP specifies a MPR test which verifies that the maximum transmit power of a user equipment is greater than or equal to the nominal total maximum transmit power minus the MPR while still complying with the ACLR requirements
As indicated above, AMPR is the Additional Maximum Power Reduction. It is band specific and is applied when configured by the network.
As can be seen from the explanations above, PCMAX is UE-implementation specific and hence not known by the eNodeB.
FIG. 25 shows exemplary scenarios for a UE transmission power status and corresponding power headroom. On the left hand side of FIG. 25, the user equipment is not power limited (positive PHR), whereas on the right hand side of FIG. 25 a negative power headroom is implying a power limitation of the user equipment. Please note that the PCMAX_L≦PCMAX≦min(PEMAX,PPowerClass) wherein the lower boundary PCMAX_L is typically mainly dependent on the maximum power reduction MPR and the additional maximum power reduction AMPR, i.e. PCMAX_L≈PPowerClass−MPR−AMPR
Further Advancements for LTE (LTE-A)
The frequency spectrum for IMT-Advanced was decided at the World Radiocommunication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved. The study item covers technology components to be considered for the evolution of E-UTRA, e.g. to fulfill the requirements on IMT-Advanced. Two major technology components which are currently under consideration for LTE-A are described in the following.
Carrier Aggregation in LTE-A for Support of Wider Bandwidth
In carrier aggregation, two or more component carriers (component carriers) are aggregated in order to support wider transmission bandwidths up to 100 MHz. All component carriers can be configured to be LTE Rel. 8/9 compatible, at least when the aggregated numbers of component carriers in the uplink and the downlink are the same. Not all component carriers aggregated by a user equipment may necessarily be Rel. 8/9 compatible.
A user equipment may simultaneously receive or transmit one or multiple component carriers depending on its capabilities. A LTE-A Rel. 10 user equipment with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple component carriers whereas an LTE Rel. 8/9 user equipment can receive and transmit on a single component carrier only, provided that the structure of the component carrier follows the Rel. 8/9 specifications.
Carrier aggregation is supported for both contiguous and non-contiguous component carriers with each component carrier limited to a maximum of 110 Resource Blocks in the frequency domain using the Rel. 8/9 numerology. It is possible to configure a user equipment to aggregate a different number of component carriers originating from the same eNodeB and of possibly different bandwidths in the uplink and the downlink:                The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the user equipment;        The number of uplink component carriers that can be configured depends on the uplink aggregation capability of the user equipment;        It is not possible to configure a user equipment with more uplink component carriers than downlink component carriers;        In typical TDD deployments, the number of component carriers and the bandwidth of each component carrier in uplink and downlink is the same.        
Component carriers originating from the same eNodeB need not to provide the same coverage.
The spacing between centre frequencies of contiguously aggregated component carriers shall be a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of Rel. 8/9 and at the same time preserve orthogonality of the subcarriers with 15 kHz spacing. Depending on the aggregation scenario, the n×300 kHz spacing can be facilitated by insertion of a low number of unused subcarriers between contiguous component carriers.
The nature of the aggregation of multiple carriers is only exposed up to the MAC layer. For both uplink and downlink there is one HARQ entity required in MAC for each aggregated component carrier. There is (in the absence of SU-MIMO—Single User Multiple Input Multiple Output—for uplink) at most one transport block per component carrier. A transport block and its potential HARQ retransmissions need to be mapped on the same component carrier. The Layer 2 structure with activated carrier aggregation is shown in FIG. 19 and FIG. 20 for the downlink and uplink respectively.
When carrier aggregation is configured, the user equipment only has one RRC connection with the network. At RRC connection establishment/re-establishment, one cell provides the security input (one ECGI, one PCI and one ARFCN) and the non-access stratum mobility information (e.g. TAI) similarly as in LTE Rel. 8/9. After RRC connection establishment/re-establishment, the component carrier corresponding to that cell is referred to as the downlink Primary Cell (PCell). There is always one and only one downlink PCell (DL PCell) and one uplink PCell (UL PCell) configured per user equipment in connected mode. Within the configured set of component carriers, other cells are referred to as Secondary Cells (SCells). The characteristics of the downlink and uplink PCell are:                The uplink PCell is used for transmission of Layer 1 uplink control information        The downlink PCell cannot be de-activated        Re-establishment is triggered when the downlink PCell experiences Rayleigh fading (RLF), not when downlink SCells experience RLF        The downlink PCell cell can change with handover        Non-access stratum information is taken from the downlink PCell.        
The reconfiguration, addition and removal of component carriers can be performed by RRC. At intra-LTE handover, RRC can also add, remove, or reconfigure component carriers for usage in the target cell. When adding a new component carrier, dedicated RRC signaling is used for sending component carriers' system information which is necessary for component carrier transmission/reception (similarly as in LTE Rel. 8/9 for handover).
When carrier aggregation is configured, a user equipment may be scheduled over multiple component carriers simultaneously, but at most one random access procedure should be ongoing at any time. Cross-carrier scheduling allows the Physical Downlink Control Channel (PDCCH) of a component carrier to schedule resources on another component carrier. For this purpose a component carrier identification field (CIF) is introduced in the respective Downlink Control Information (DCI) formats. A linking between uplink and downlink component carriers allows identifying the uplink component carrier for which the grant applies when there is no-cross-carrier scheduling. The linkage of downlink component carriers to uplink component carriers does not necessarily need to be one to one. In other words, more than one downlink component carrier can link to the same uplink component carrier. At the same time, a downlink component carrier can only link to one uplink component carrier.
(De)Activation of a Component Carrier and DRX Operation
In carrier aggregation, whenever a user equipment is configured with only one component carrier, LTE Rel. 8/9 discontinuous reception (DRX) applies. In other cases, the same DRX operation applies to all configured and activated cells, respectively component carriers (i.e. identical active time for PDCCH monitoring). When in active time, any component carrier may always schedule Physical Downlink Shared Channel (PDSCH) on any other configured and activated component carrier (further restrictions are free for study).
To enable reasonable user equipment battery consumption when carrier aggregation is configured, a component carrier activation/deactivation mechanism for downlink SCells is introduced (i.e. activation/deactivation does not apply to the PCell). When a downlink SCell is not active, the user equipment does not need to receive the corresponding PDCCH or PDSCH, nor is it required to perform CQI measurements (CQI is short for Channel Quality Indicator). Conversely, when a downlink SCell is active, the user equipment shall receive PDSCH and PDCCH (if present), and is expected to be able to perform CQI measurements. In the uplink however, a user equipment is always required to be able to transmit on PUSCH on any configured uplink component carrier when scheduled on the corresponding PDCCH (i.e. there is no explicit activation of uplink component carriers).
Other details of the activation/deactivation mechanism for SCells are:                Explicit activation of downlink SCells is done by MAC signaling;        Explicit deactivation of downlink SCells is done by MAC signaling;        Implicit deactivation of downlink SCells is also possible;        downlink SCells can be activated and deactivated individually, and a single activation/deactivation command can activate/deactivate a subset of the configured downlink SCells;        SCells added to the set of configured component carriers are initially “deactivated”.        
Uplink Power Control for Carrier Aggregation
Even though most details of the uplink power control algorithm for the carrier aggregation case are still open or under discussion in the 3GPP working groups, the general agreement is that LTE-A Rel. 10 supports component carrier specific uplink power control, i.e. there will be one independent power control loop for each uplink component carrier configured for the user equipment. Furthermore it was decided that power headroom should be reported per-component carrier. In case of power limitation, i.e. UE transmission power is exceeding the total maximum UE transmit power, the following power scaling is applied.
For power scaling, the PUCCH power should be prioritized and the remaining power may be used by PUSCH (i.e. PUSCH power is scaled down first, maybe to zero). Further, a PUSCH with uplink control information (UCI) is prioritized over PUSCH without UCI, Additionally, equal power scaling for PUSCH transmissions without UCI is considered.
As each component carrier can be assumed to have its own power control loop and each transport block on each component carrier is transmitted with a power individually set for the component carrier, power headroom reporting should be performed per component carrier. Since carrier aggregation can be seen as a multiplication of several LTE Rel. 8/9 (component) carriers, it can be assumed that also the power headroom reporting on the individual component carriers will reuse the LTE Rel. 8/9 power headroom reporting procedures.
Hence each user equipment transmits power headroom reports for each component carrier on that component carrier. This means that each component carrier that has an uplink transmission in a specific sub-frame could also transmit a power headroom report given that the conditions for sending such a report are fulfilled.
Power headroom reporting as know form LTE Rel. 8/9 is controlled, respectively triggered on a component carrier basis (by employing different timers). Applying this concept to the individual component carriers of a system utilizing carrier aggregation, this means that it almost never happens that within one sub-frame each of the component carriers with an uplink transmission is transmitting a power headroom report. Hence, even if the timers relating to power headroom reporting (the periodicPHR timer and the prohibitPHR timer) are set to the same values for all component carriers, synchronous power headroom reports on all component carriers within a sub-frame will only happen by chance.
FIG. 10 shows exemplary power headroom reporting in a LTE-A system, assuming that the power headroom reporting of LTE Rel. 8/9 is applied to each of the exemplary three component carriers (CoCa1 to CoCa3). At T1, there is an uplink assignment on all three component carriers and an uplink transport block, respectively MAC PDU, including a power headroom report for the respective component carrier is sent on each component carrier. As there is a per-component carrier (per-CC) power headroom report for each component carrier, the eNodeB is informed on the user equipment's power status. Furthermore, the respective timers periodicPHR-Timer and prohibitPHR-Timer are restarted for each component carrier. For component carriers CoCa2 and CoCa3, it is assumed that after expiry of the periodicPHR-Timer there is no uplink allocation in the next sub-frame, so that no periodic power headroom report can be sent immediately. Hence at T2, the user equipment transmits a transport block/MAC PDU with a power headroom report only on component carrier CoCa1. As there is only a resource assignment on component carrier CoCa1, the eNodeB may again conclude on the user equipment's power status from the per-CC power headroom report at T2.
However at T3, T4, and T5, only some transport blocks/PDUs of the component carriers within a sub-frame carry a power headroom report. Regarding the power headroom report on component carrier CoCa3 at T5. A path-loss change on component carrier CoCa3 is assumed to trigger the power headroom report, but at the time of the path-loss change none of the component carriers with uplink transmissions (i.e. component carriers CoCa1 and CoCa2) have a power headroom report included. Therefore, T3, T4, and T5, the eNodeB is not aware of the actual transmit power spend on the uplink transmissions within the respective sub-frames.
Furthermore, in LTE Re1.10 within the scope of carrier aggregation there are two maximum power limits, a total maximum UE transmit power PCMAX and a component carrier-specific maximum transmit power PCMAC,c. 3GPP RAN4 working group already indicated that both (nominal) maximum transmit power per user equipment PCNMAX and the (nominal) maximum component carrier-specific transmit power PCMAC,c should be the same regardless of the number of carriers supported, in order not to affect the link budget of a carrier aggregation capable user equipment in the single carrier operation mode.
Different to LTE Rel. 8/9, in LTE-A Rel. 10 the user equipment has also to cope with simultaneous PUSCH−PUCCH transmission, multi-cluster scheduling, and simultaneous transmission on multiple component carriers, which requires larger MPR values and also causes a larger variation of the applied MPR values compared to 3GPP Rel. 8/9.
It should be noted that the eNodeB does not have knowledge of the power reduction applied by the user equipment on each component carrier, since the actual power reduction depends on the type of allocation, the standardized MPR value and also on the user equipment implementation. Therefore eNodeB doesn't know the component carrier-specific maximum transmission power relative to which the user equipment calculates the power headroom. In LTE Rel. 8/9 for example the user equipment maximum transmit power PCNMAX can be within some certain range as described above (PCMAX_L≦PCMAX≦PCMAX_H).
Due to the reduction of the component carrier-specific maximum transmission power PCMAC,c, which is not known to eNodeB as explained above, the eNodeB cannot really know how close a user equipment is operating to its total maximum transmission power PCNMAX. Therefore there might be situations where user equipment is exceeding the total user equipment maximum transmission power PCNMAX which would hence require power scaling. FIG. 26 shows an exemplary scenario where user equipment is power limited, i.e. applying power scaling on component carriers CC#1 and CC#2 configured in the uplink. Even though the user equipment is power limited, the component carrier-specific power headroom reports according to the LTE definitions indicate sufficiently large power headroom.